Here there should be made a clear distinction between a watch case including a caseband blocked by a back cover of large diameter where the back cover must be removed to have access to the battery, and a watch case including a caseband-back provided with a simple cover giving access to the battery.
The first of these arrangements is described for instance in the European Pat. No. EP-B-0 024 364 corresponding to U.S. Pat. No. 4,362,396. In this construction the movement is mounted from below the caseband, which itself is blocked by a screwed in back cover or by one which is pressed on. The back cover must be removed in order to proceed to the changing of the battery and this operation can only be accomplished by a person skilled in the art equipped with special tools and who at the same time will take care to retain the movement so that the latter does not fall out of the caseband. This arrangement provides the advantage of taking up very little space in the thickness of the watch since the back cover is attached to the caseband at a place situated above the lower face of the movement. However, as has just been noted, the battery is not accessible to anybody which may be considered as a disadvantage, above all at the present time, when certain inexpensive watches regularly offer this possibility.
The second arrangement is set forth for instance in the Japanese utility models JP Nos. 1 131 545 and 1 231 593. In these the watch case is provided with a caseband back, sometimes referred to as a monoshell case, wherein the movement is mounted from above the caseband. The cited documents show that the caseband is provided with an enlarging ring and supports for retaining the movement. There is provided in the cover of said caseband-back an opening just sufficient to enable the changing of the battery. This opening is blocked by a cap screwed into the back. This manner of proceeding provides the advantage of simplifying the assembly of the watch on automatic or transfer machines. Effectively all the loading operations take place from the same side, i.e. the top of the watch, and it is not necessary to turn over the timepiece in order to provide it with the various components. This second arrangement also provides the advantage of a back which takes up little space in the thickness. Furthermore, the battery cover may be removed by anybody and without requiring special tools. Generally, the cover provides a slot in which may be introduced a coin. The details of such a construction are shown in the British Pat. No. GB-A-1 408 610. It is immediately seen however that this cover takes up considerable space in thickness initially because of the place which must be provided for the slot receiving the coin and subsequently because of the thickness which must be provided for the screwthreads and the watertight packing. Finally, this method of proceeding is inelegant because of the outward projection of the cover.
To overcome the difficulties which have just been cited, the British Pat. No. GB-A-1 568 475 proposes to provide a cap of large diameter provided with a slot thanks to which the cap may be assembled for instance with the aid of a coin. In this document the groove is no longer situated under the battery, but beside the latter in a manner such that the thickness over the battery of the movement is not increased by the presence of this slot. However, the position of said slot is such that it requires a free space between the movement and the inside of the cap, such space finally resulting in an increase in the thickness of the watch in all cases where the thickness of the battery is included substantially in the thickness of the movement. It will be seen from the description of the present invention that no space is provided between the cap and the movement since the means of unscrewing the cap are situated outside the periphery of such movement, this enabling reduction to a minimum of the thickness of the watch.
To facilitate the construction as suggested in the description to follow, there will be preferably employed a bayonet-type lock, such type of lock being similar to that described in the Swiss Pat. No. CH-A-374 937 where the fastening means of the cap to the caseband are located at a level above the lower face of the movement. In this patent however, there is provided a watch case with a mechanical movement wherein the back cover must be removable, not for changing a battery, but in order to assemble or repair the movement and thus the access to the interior is not possible except to a person skilled in the art provided with special tools as has already been mentioned with reference to the first embodiment mentioned in the introduction to the present application and as has been described in the cited patent wherein the utilization of a press is mentioned. Furthermore, in this patent, the wish to provide a watch of small thickness is not apparent since considerable space remains between the back cover and the movement.